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Cofferdam types, uses, merits and demerits

Short answer

A cofferdam is a temporary enclosure to exclude water, enabling dry construction. Earthen types suit shallow, slow water at low cost; single-wall braced piles offer economy for moderate depths but struts obstruct work; double-wall types are stronger for greater depth; cellular cofferdams are self-supporting, reusable structures for deep water, though more expensive.

What is a cofferdam? Describe with neat sketches the different types of cofferdam, their uses and their relative merits and demerits.

A cofferdam is a temporary watertight enclosure built in water-bearing ground (river, lake, sea or wet soil) from which the water is pumped out so that foundations, piers or other structures can be built in dry conditions.

Types, sketches and uses

1. Earthen (and rockfill) embankment cofferdam

   ~~~ water ~~~\
                 \___________
                  /  clay core \
   ______________/______________\____

Used in shallow, slow rivers and low water depth (up to 3 m) for dams and bridge piers.

2. Single-wall (braced) sheet pile cofferdam

   ~~~ water   |sheet|====strut====|sheet|  water ~~~
               |pile |             |pile |
               |_____|  dry area   |_____|

A ring of interlocking sheet piles braced by wales and struts; used for piers and small foundations in water up to about 6-10 m deep.

3. Double-wall cofferdam

   | sheet pile |  sand/gravel fill  | sheet pile |
   |            |  (tied by rods)    |            |

Two parallel sheet-pile walls tied together with a gravel or sand fill between; used for deeper water (up to about 12 m).

4. Cellular cofferdam (circular, diaphragm or cloverleaf cells of steel sheet pile filled with sand/gravel)

    ()  ()  ()   (a row of circular cells joined by arcs,
   (  )(  )(  )   filled with granular soil)

Used for large works, dams, locks and deep water (up to about 25 m); self-supporting without internal bracing.

5. Crib (timber or concrete) and other special types: boxes or cribs filled with stones; used in rocky riverbeds.

Relative merits and demerits

TypeMeritsDemerits
Earth/rockfillCheap, simple, local materialNeeds large area; leaks; unsuitable for deep or fast water
Single-wall bracedEconomical, quick for small sitesStruts obstruct work; limited depth; leakage at joints
Double-wallStronger, good for greater depth, less leakageCostly, requires much material and space
CellularNo bracing, large clear working area, reusable, good in deep waterHigh cost, needs skilled work, needs rock/stiff base to avoid scour and slip, failure if a cell breaks

✓ Answer checked by Claude Sonnet 5.5

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